The present invention relates to hydraulic systems and pertains particularly to a system having a speed override valve which automatically causes the speed control valve of the system to shift to its low-speed position when the load on the hydraulic motor exceeds a predetermined value.
Some rotary output radial piston hydraulically driven motors are designed for multi-speed operation by selective direction of all fluid flow through a pair of inlet ports for high-torque low-speed operation or through only one of the inlet ports for low-torque high-speed operation. Such hydraulic motors are commonly employed in industrial and earth-moving installations such as winches and hoists or pipelayers and the like. One problem associated with hydraulic systems employing such motors is that excessive hydraulic pressure exceeding the rated pressure may be generated in the motor if the high-speed mode is engaged to raise a suspended load which is too heavy for the high-speed mode. Such excessive pressures may cause the fluid to leak past the pistons of the motor and could permit unexpected dropping of the suspended load. Such a situation could present a hazardous situation. Likewise excessive pressure may be generated if the proper speed mode is not selected for lowering such heavy loads.
One approach to multi-speed hydraulic motor drives is illustrated in the following U.S. patents: U.S. Pat No. 3,768,263 issued Oct. 30, 1973; U.S. Pat No. 3,610,507 issued Oct. 5, 1971; and U.S. Pat. No. 3,473,442 issued Oct. 21, 1969. These disclosures teach the use of a plurality of motors driven in parallel for low-speed high-torque operation and driven in series for high-speed low-torque operation.
However, none of these disclose a speed override control system that is operative to override the speed selected should the load be excessive for that speed.